This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Objective: To develop a low dose challenge model using SIVsmE660 in rhesus macaques. The recent failure of the HIV Vaccine Trial Network and Merck's STEP trial has renewed interest in testing HIV vaccine concepts in non-human primates prior to large-scale human trials. There are many difficulties in developing a successful HIV vaccine but a major hurdle is the enormous amount of viral diversity. To address this issue several strategies have been proposed to improve vaccine design by including immunogens based on ancestral, center of tree, or consensus HIV-1 sequences. It is estimated these methods can limit the difference between clade-specific vaccines and infecting virus to 5-15% depending on the protein. In the Rhesus macaque model of HIV infection the molecularly cloned virus SIVmac239 is commonly used as the basis for vaccine constructs. Until recently SIVmac239 or the substantially similar SIVmac251 were often used to test vaccine efficacy. These homologous virus challenges, however, provided an artificially non-demanding pre-clinical trial. Therefore, to adequately test vaccine concepts in non-human primates a physiologically relevant heterologous virus challenge model is needed. We developed a more physiologically relevant challenge model by administering multiple "low" doses of SIVsmE660 as a heterologous challenge for SIVmac239-based vaccines in pre-clinical trials. SIVsmE660 is a "swarm" virus containing numerous quasispecies within the viral stock, which is analogous to the quasispecies circulating in HIV infected humans. The consensus amino acid sequence of our SIVsmE660 stock is approximately 15% different than SIVmac239. This variation is approximately the predicted difference between clade-based HIV vaccines and circulating strains of that clade. In humans it has recently been discovered that only a few HIV viruses are able to establish infection. Preliminary analysis of our "low" dose challenge model suggests that intrarectal inoculation of macaques with 6-12 million SIVsmE660 virions mimics the findings in humans by resulting in only one to three virus particles establishing infection. The research uses WNPRC Immunogenetics &Virology Services.